In the manufacture of nuclear fuel for light water reactors, tubes of usually a zirconium alloy, such as Zircaloy-2 or Zircaloy-4, are provided with an end closure, whereupon the tube is filled with fuel pellets to the desired length. After that, the rod is pressurized with helium before it is completely sealed. By filling the fuel rod with helium, the heat transfer from the pellet out towards the coolant is improved.
During the sealing of the fuel rod, the rod is usually provided with plugs of a zirconium alloy which are designed such that part of the plug may make contact with the tube end of the rod, and by welding around the joint between tube and plug, the rod is sealed. A number of different welding methods, such as TIG welding, EB welding or resistance welding, are used in this connection. When using welding methods which do not require vacuum, for example TIG and resistance welding, pressurization of the rod with helium can be carried out in connection with the plug being welded on. For welding methods which require vacuum, for example EB welding, pressurization of the rod must be carried out separately. This is done by providing the plug with a fill hole through which helium may be supplied to the rod and which is thereafter sealed by means of TIG welding.
The disadvantages of the prior art methods of sealing a fuel rod after pressurization are that TIG welding functions badly in helium environments. High voltages are required for igniting the welding arc, and welding electrodes are consumed very rapidly and have to be replaced. Since this type of manufacturing usually takes place in continuous lines, frequent replacements of welding electrodes entail considerable drawbacks.
Conventional resistance welding, of the joint between the plug and the tube end of the rod entails problems in that the type of defects which may occur at the weld joint after resistance welding, such as, e.g., oxide streaks, cannot be controlled by some form of non-destructive testing. Since nuclear fuel rods operate at high pressures and contain substances, the spreading of which outside the rod is not desirable, it is of the utmost importance that sealing welds do not contain defects which may lead to continuous cracks in the weld.